An optical fiber sensor or an optical fiber sensor system which has attracted much interest these days is characterized in that all or a part of it is made of an optical fiber, and it can be divided into an intrinsic type in which a sensing unit itself for measuring a physical amount is made of an optical fiber and an extrinsic type in which the optical fiber only serves as a light source and a means for delivering an optical signal.
Generally, the optical fiber sensor or the optical fiber sensor system uses light for measuring, and therefore it does not consume electricity differently from an electric sensor based on a semiconductor or a conductive material. Therefore, dangers such as a noise due to an electromagnetic wave interference generated from around, a breakage due to electric leakage, and an electric shock, etc. can be prevented for the optical fiber sensor or the optical fiber sensor system. Also, the optical fiber sensor or the optical fiber sensor system is small, lightweight, and has a good sensitivity. In this regard, the optical fiber sensor or the optical fiber sensor system can be utilized for measuring various types of physical amounts. For example, the optical fiber sensor can measure a variation in various physical amounts such as a temperature, a strain, a bending, a torsion, a pressure, a refractive index, a concentration, a PH, an optical power, a current, a voltage, etc.
The optical fiber sensor has a better environmental resistance and a longer lifetime than an electrical sensor, has a superb reproducibility due to a small plastic deformation, and enables a long-distance measuring system to be developed since it uses light as a measuring means. Also, it is possible to develop a high speed mass capacity measuring system using a single line by adopting a wavelength division multiplexing (WDM) technology and a time division multiplexing (TDM) technology. Although it had been expected that the optical fiber sensor would replace conventional electrical sensors very quickly thanks to these advantages, advances in commercialized technologies in optical characteristic analysis and measurement fields are relatively small, which acts as an obstacle in expanding related markets.
The optical fiber sensor system can mainly be configured with a light source unit serving as a measuring light source, a sensing unit which varies a characteristic of light (an input beam) delivered from the light source unit according to a physical amount applied from an external environment, and a measuring unit which measures and analyzes the characteristic of the light varied by the sensing unit. In particular, in the measuring unit, a detecting unit for detecting the light and a signal analysis device for deriving the physical amount play an important role in the performance of the overall sensor system.
Among various optical fiber sensor systems, a scheme which applies an optical fiber Bragg grating (FBG) to the sensing unit and analyzes a shift of an optical wavelength generated in the FBG to measure the physical amount is widely used. That is, when the optical fiber Bragg grating (FBG) is used for the sensing unit, the FBG reflects a portion of the input beam delivered from the light source unit to generate a band type optical signal with a constant width, and configures the sensor system by using a movement characteristic of a central wavelength (or a resonance wavelength) of this optical band. Also, in this category, there is a sensor system using a long period optical fiber (LPFG), a Fabry-Ferot filter, and an optical element utilizing optical phenomena such as a Brilliouin scattering, a stimulated Brilliouin scattering (SBS), a Raman scattering a Mie scattering, a coherent anti-stokes Raman scattering (CARS), an optical parametric generation, a sum frequency generation (SHG, THG), a difference frequency generation, a four wave mixing (FWM), etc.
That is, all optical signals are made in band shapes and central (resonance) wavelengths of the bands are moved according to an applied physical amount such as a temperature, a strain, a refractive index, etc. Then the moved central wavelengths of the bands are measured to analyze a variation value of the physical amount. Although the optical element for a sensing unit used in the optical fiber sensor system should not necessarily be configured as an optical fiber, it is preferred that the whole part or at least an input unit and an output unit of the optical element are made as an optical fiber when considering an optical connectivity with the optical fiber element configuring the rest part of the optical fiber sensor system. In case of the optical fiber sensor system using various optical elements as the sensing unit, since a band type optical signal (signal beam) is generated in the sensing unit and a shift of a central wavelength of the signal beam due to an environment variation is measured to derive the physical amount, a measuring device (an interrogation device), which detects a wavelength variation and derives the physical amount from the detected wavelength variation, has to be included in the sensor system.
In order to accomplish this, according to the prior art, various optical characteristic analysis devices using an interrogation technology based on a bulk optic filter, an optical fiber coupler, and an optical fiber grating, as well as an optical fiber sensor technology using the same have been developed.
FIG. 1 shows an optical fiber sensor system based on an optical characteristic measuring device which uses a bulk optic filter element according to the prior art. In the prior art as shown in FIG. 1, a bulk optic filter element such as an edge Filter and a band pass filter having a transmission characteristic with a constant slope is used as the interrogation element. The optical fiber sensor system which uses the bulk optic filter element such as the edge filter and the band pass filter needs an optical coupling between the bulk optic filter element and different optical fiber elements in order to reproduce necessary optical characteristics. The prior art has a drawback in that a precise optical alignment procedure, which requires much time and cost, and an additive optical component such as a collimator are needed to realize the optical coupling.
Furthermore, additional components such as a housing are required to protect the optical fiber components configured with bulk optics from contamination and external vibrations and to realize a stable optical characteristic, which makes an overall configuration complex. Generally, the bulk optic filter element also has drawbacks in that it is manufactured by depositing plural thin films on a substrate material by using a high vacuum deposition process, and it requires various high cost processes since an anti-reflection coating is needed to reduce an optical loss due to the reflection occurring on a surface exposed to air.
Also, after the bulk optic filter is manufactured, it is impossible to further adjust a thickness of the element for controlling the light absorption (optical absorption) characteristic (for example, a light absorption intensity and a slope) in order to obtain a precise interrogation performance, and, therefore, the filter element has to be manufactured individually according to the specification of the different sensor system.
FIG. 2 shows an optical fiber sensor system based on an optical characteristic measuring device which uses an optical fiber coupler according to the prior art. As shown in FIG. 2, an optical fiber coupler having a transmission spectrum with a constant slope can be used as an interrogation element as another interrogation method for measuring optical characteristics. In this optical characteristic analysis system adopting the optical fiber coupler as above, a coupled signal and a transmissed signal cross each other to improve sensitivity. However, it is very hard to manufacture an optical coupler which has the optical characteristic with a linear optical absorption slope and can precisely represent this optical characteristic.
In general, also, since the optical characteristic of the optical fiber coupler is easily affected by an external environment such as a polarization characteristic of light, a temperature, a vibration, etc., a complex device configuration and an additional process technology are required to solve this problem, which results in drawbacks.
FIG. 3 shows an optical fiber sensor system based on an optical characteristic measuring device which uses a long period fiber grating according to the prior art. As shown in FIG. 3, an optical characteristic analysis device adopting a transmission spectrum characteristic of a long period optical fiber grating having a slope is supposed as another interrogation method for measuring the optical characteristics.
In this method as above, since the sensor system is configured as an optical fiber, a problem due to an optical alignment can be solved; however, it is very hard to manufacture optical fiber gratings having the same characteristics with high reproducibility. Also, since an optical characteristic itself of the long period optical fiber grating for interrogation, which is supposed to be stable with respect to peripheral environment, is generally very sensitive to a vibration and a temperature and shows a polarization dependent characteristic, it is very difficult to commercialize this type of sensor system.